The innate immune response is responsible for the early recognition and control of infection. Pathogen recognition by the innate immune system is principally via conserved pattern recognition receptors that activate the cellular components of the response such as macrophages. An effective outcome is to amplify the response to pathogens, provide early control of pathogen numbers and spread, and push the immune system toward the development of adaptive immunity. The innate immune system is therefore essential for immunity to virtually all pathogens to which we are exposed. However, the pro-inflammatory response must be carefully regulated or dire consequences result for the host. The clearest consequence of an overactive innate immune response is sepsis. In this poorly understood disease, runaway systemic cytokine production from pathogen-activated macrophages leads directly to hypotension and multiorgan failure. Overactive innate immune responses also play a role in chronic inflammatory diseases. The goals of this proposal are to investigate the biology of a new described group of molecules termed the SOCS (Suppressor of Cytokine Signaling) proteins in down-regulating activated macrophages. SOCS protein-containing complexes are ubiquitin E3 ligases that attach polyubiquitin chains to target proteins in order to direct them for degradation by the proteosome. Genetic and biochemical evidence suggest that both direct inhibition of signaling and promoting degradation are likely to be important in the ability of SOCS proteins to regulate signaling. Substantial evidence implicates SOCS proteins in key decision making events in the inflammatory response. This proposal addresses the role of SOCS3 and SOCS1 in regulating signal transduction in activated macrophages. Aim 1 will address the role of SOCS3 in macrophage deactivation through the use of macrophages that lack SOCS3 along with mice that lack SOCS3 in their hematopoietic system. Aim 2 will determine the substrates of SOCS3 in pathogen-challenged macrophages to gain insight into which proteins SOCS3 targets for degradation during deactivation. Aim 3 will investigate the regulation of the SOCS3 gene in response to pathogen challenge. Together, these studies will provide new insights into macrophage function, SOCS protein biology and the regulation of the innate immune response. PERFORMANCE SITE (S) (organization, city, state) St. Jude Children's Research Hospital, Memphis, TN .